1. Field of the Invention
The present invention relates to a finder optical system in a camera. More precisely, the invention relates to a ghost image preventing optical system in a finder optical system which is separate from a photographing optical system.
2. Description of Related Art
Recently, finder optical systems or photographing optical systems having synthetic resin lenses have been widely used. This is due to the fact that synthetic resin lenses are inexpensive and can be freely and easily formed into a desired shape, such as an aspherical lens.
In a single lens reflex camera, the photographing optical system and the finder optical system are constituted by the same optical system. The photographing optical system is usually provided with an anti-reflection layer (hereinafter referred to as AFL) to prevent a ghost image from occurring. Consequently, in a single lens reflex camera, a ghost image will not occur in the finder optical system.
However, in a camera in which the photographing optical system is provided separately from the finder optical system, the same would not be true. The photographing optical system is subject to anti-reflection surface treatment to prevent a ghost image, similar to the single lens reflex camera, but nothing is done to prevent a ghost image from occurring in the finder optical system. The reason for this being that when light emitted from an eyepiece (i.e., ocular lens) in the finder optical system is incident upon viewer's eyes, the viewer usually does not see a ghost image, since the human eye naturally eliminates most ghost images. Synthetic resin lenses usually have a refractive index of 1.5.about.1.6 lower than other common optical elements. Consequently, in the finder optical system having a small reflectance and made of synthetic resin lenses, the ghost image is usually not apparent.
While it is unlikely that ghost images will appear in finder optical systems as described above, it cannot be assumed that these images will be absent in all cases. On the contrary, ghost images have been known to occur in recent real image type finder optical systems. According to a study conducted by the inventor of the present invention, a distinctive ghost image is likely to occur when a real image type of finder optical system has a large number of lens surfaces. In recent real image type finder optical systems, the lenses are separate so that the finder can be miniaturized and the power variability increased. Such a finder optical system is also beneficial for mass production. However, the number of lens surfaces in this type of finder optical system is greater than conventional finder optical systems in which the lenses are adhered to one another. Furthermore, the diameter of the lenses on the objective side in recent real image type finder optical systems is made larger than that of conventional finder optical systems to realize a wider angle lens. This results in light from outside of the field of view falling into the finder optical system, particularly at high magnification (i.e., high power).